How can I become a volunteer therapist for ptsd with our veterans?

You can volunteer. You certainly can volunteer and the military would be happy for your service! i would suggest contacting the national guard bureau and discussing options. Good luck!
Call the VA. Call your local VA hospital and ask about volunteering opportunities to help veterans.
Case Foundation. The case foundation is one organization through which trained mental health professionals can provide pro bono services to returning vets and their families. They provide an hour a week, and the people receiving these services also have the opportunity to give back an hour of volunteer time in their own community.